The feasibility of using a nuclear reactor as a thermally radiating energy source is investigated. Design parameters including radiant energy transmission, attenuation of γ and neutron fluxes, criticality, and heat transfer are evaluated. Since conclusive experimental data for nuclear and high temperature heat transfer properties are lacking, a mathematical relationship between these limiting parameters is derived and the results are presented for the whole range over which they are likely to vary. For some of the designs considered, the overlap within which a thermally radiating nuclear reactor can be considered feasible is sufficiently broad to cover all possible uncertainties. It is concluded, therefore, that a spherical W-UO2 cermet shell reactor, operating at a surface temperature of 2800°K, assuming a modest extrapolation of present-day materials technology, is indeed feasible.